C++: Add test for meson configuration files

Author: jketema

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/meson-private/tmp_abc/ExprHasNoEffect.expected

@@ -0,0 +1 @@
+| testfile.c:6:3:6:8 | call to strlen | This expression has no effect (because $@ has no external side effects). | testfile.c:3:8:3:13 | strlen | strlen |

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/meson-private/tmp_abc/ExprHasNoEffect.qlref

@@ -0,0 +1 @@
+Likely Bugs/Likely Typos/ExprHasNoEffect.ql

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/meson-private/tmp_abc/testfile.c

@@ -0,0 +1,8 @@
+typedef long long size_t;
+
+size_t strlen(const char *s);
+
+int main() {
+  strlen(""); // GOOD: the source file occurs in a `meson-private/tmp.../testfile.c` directory
+  return 0;
+}